Automotive vehicles include a powertrain that is comprised of an engine, a multi-speed transmission, and a differential or final drive. The multi-speed transmission increases the overall operating range of the vehicle by permitting the engine to operate through its speed range a number of times. The number of forward speed ratios that are available in the transmission determines the number of times the engine speed range is repeated. Early planetary gear transmissions had two speed ranges. This severely limited the overall speed range of the vehicle and required a relatively large engine that could produce torque through a wide speed range. This resulted in the engine operating at a specific fuel consumption point during cruising, other than the most efficient point. Therefore, more expensive, countershaft transmissions with three or four speed ratios became more popular.
With the advent of three- and four-speed automatic transmissions, the planetary gear transmission increased in popularity with the motoring public. These transmissions improved the operating performance and fuel economy of the vehicle. The increased number of speed ratios reduces the step size between ratios and therefore improves the shift quality of the transmission by making the ratio interchanges substantially imperceptible to the operator under normal vehicle acceleration.
A variety of different types of transmissions are used to deliver multiple speed ratios including manual, automated manual, dual clutch and planetary transmissions. For example, a typical multi-speed, dual clutch transmission uses a combination of two friction clutches and several dog clutch/synchronizers to achieve “power-on” or dynamic shifts by alternating between one friction clutch and the other, with the synchronizers being “pre-selected” for the oncoming ratio prior to actually making the dynamic shift. This concept typically uses intermediate shaft gears with a different, dedicated gear pair to achieve each forward speed ratio (with the exception of some transmissions being able to achieve a direct drive ratio in a front engine, rear wheel drive application). The transmission utilized for a specific application may depend on many factors, such as a minimization of required components, packaging limitations, ratio coverage and torque requirements for launch.
Dual-clutch transmissions (DCTs) are, operationally, two manual-type transmission modules acting in parallel. DCTs typically shift in an alternating pattern, so that one module carriers torque while the other is selecting the next speed ratio. In this way, DCTs can transmit torque at all times, without interruption. However, they include the complexity, mass and bulk of two transmission modules, one of which is unused at any given time. Furthermore, since DCTs shift in an alternating pattern, almost half of the ratios are unavailable for selection at any given moment. That is, for example, a conventional DCT cannot shift directly from its sixth speed ratio to its second or fourth speed ratio without torque interruption.